As is well known, discharge of refrigerants into the atmosphere is considered to be a major cause of the degradation of the ozone layer. While refrigerants such as R134a are certainly more environmentally friendly than refrigerants such as R12 which it replaced, they nonetheless are undesirable in that they may contribute to the so-called greenhouse effect.
Both R12 and R134a have been used largely in vehicular applications where weight and bulk are substantial concerns. If a heat exchanger in an automotive air conditioning system is too heavy, fuel economy of the vehicle will suffer. Similarly, if it is too bulky, not only may a weight penalty be involved, but the design of the heat exchanger may inhibit the designer of the vehicle in achieving an aerodynamically "slippery" design that would also improve fuel economy.
Much refrigerant leakage to the atmosphere occurs from vehicular air-conditioning systems because the compressor cannot be hermetically sealed as in stationary systems, typically requiring rotary power via a belt or the like from the engine of the vehicle. Consequently, it would be desirable to provide a refrigeration system for use in vehicular applications wherein any refrigerant that escapes to the atmosphere would not be as potentially damaging to the environment and wherein system components remain small and lightweight so as to not have adverse consequences on fuel economy.
These concerns have led to consideration of transcritical CO.sub.2 systems for potential use in vehicular applications. For one, the CO.sub.2 utilized as a refrigerant in such systems could be claimed from the atmosphere at the outset with the result that if it were to leak from the system in which it was used back to the atmosphere, there would be no net increase in atmospheric CO.sub.2 content. Moreover, while CO.sub.2 is undesirable from the standpoint of the greenhouse effect, it does not affect the ozone layer and would not cause an increase in the greenhouse effect since there would be no net increase in atmospheric CO.sub.2 content as a result of leakage.
Such systems, however, require the use of a suction line heat exchanger to increase the refrigerating effect of the evaporator due to thermodynamic property relationships. If not used, an unusually high mass-flow rate of CO.sub.2 and correspondingly high compressor input power levels are required to meet typical loads found in automotive air conditioning systems. Through the use of a suction line heat exchanger, the CO.sub.2 mass-flow rate and compressor input power may be lowered with the expectation that a reduction in the size of the system compressor may be achieved. At the same time, the addition of a suction line heat exchanger to the vehicle has the potential for increasing weight as well as to consume more of the already limited space in the engine compartment of a typical vehicle. Thus, there is real need for a highly compact, highly effective suction line heat exchanger.
Heretofore, suction line heat exchangers have been utilized in relatively large refrigeration systems where the refrigerant discharged from the evaporator must be passed as a super-heated vapor to the compressor to assure that no liquid enters the compressor. This is necessary as compressors conventionally employed in refrigeration systems are positive displacement devices. As such, if any liquid refrigerant, coexisting within gaseous refrigerant in a saturated state, were drawn into the compressor, severe damage and/or loss of compressor pumping capacity would be likely to result.
Suction line heat exchangers avoid the difficulty by bringing, relatively hot, condensed refrigerant from the outlet of the system condenser or gas cooler into heat exchange relation with the refrigerant being discharged from the evaporator at a location between the evaporator and the compressor. As a consequence, the refrigerant stream exiting the evaporator will be heated. The suction line heat exchanger is sized so that the stream ultimately passed to the compressor from the suction line heat exchanger is a super-heated vapor at a temperature typically several degrees above the saturation temperature of the refrigerant at the pressure at that point in the system. As a consequence, no refrigerant will be in the liquid phase and the compressor will receive only a gaseous refrigerant. A typical system of this sort is shown schematically in FIG. 1.
Conventional suction line heat exchangers for commercial refrigeration applications are usually concentric, round tube devices having a substantial length. They are not suited for applications where space is at a premium. Other forms of suction line heat exchangers include the use of a large diameter round tube for conducting the outlet stream of the evaporator to the compressor. This tube is wrapped with a smaller diameter round tube which is employed to conduct liquid refrigerant from the condenser to the expansion device of the system. This form of heat exchanger is somewhat of an improvement over concentric round tube structures in that it takes the place of some of the connecting conduit between the condenser and the expansion device on the high-pressure side and between the evaporator and compressor on the low-pressure side, thereby providing somewhat of a space savings. However, the same remains quite bulky and consequently would be unsuitable for use in mobile refrigeration systems as, for example, vehicular air conditioning systems.
To achieve compactness, it has been proposed to combine the evaporator and the suction line heat exchanger into a single unit. One example of such a construction is shown in U.S. Pat. No. 5,678,422 issued Oct. 21, 1997 to Yoshii et al. Proposed is a so-called drawn cup evaporator construction which, at one end, is provided with a further drawn cup type of heat exchanger which serves as a suction line heat exchanger. While some degree of compactness is achieved, the addition of the drawn cup suction line heat exchanger adds considerable bulk to the evaporator.
Another instance of integrating a suction line heat exchanger in an evaporator is illustrated is U.S. Pat. No. 5,212,965 issued May 25, 1993 to Datta. In this patent, there is disclosed a round tube, plate fin type of evaporator construction which itself is relatively bulky with the consequence that sizable volume reductions cannot be obtained in spite of the integration of the suction line heat exchanger with the evaporator.
Kritzer in U.S. Pat. No. 3,274,797 issued Sep. 27, 1966 discloses a vapor compression refrigeration system, typically used in refrigeration, bringing a capillary tube interconnecting a condenser and evaporator (presumably serving as an expansion device) into contact with the suction line of the compressor to achieve heat exchange therebetween. Kritzer states that this varies the flow rate of refrigerant to the evaporator in response to the temperature of the refrigerant in the suction line to the compressor. While it thus appears that Kritzer is concerned with the exchange of heat between the outlet stream of the evaporator and the inlet stream from the condenser at the expansion device, it is done for the purpose of achieving flow control and therefore is not a suction line heat exchanger in the conventional sense.
Vakil in U.S. Pat. No. 4,304,099 issued Dec. 8, 1981 is somewhat similar in that a capillary tube connected to the outlet of the condenser is brought into heat exchange contact with an external surface of the evaporator along its entire length before discharging into the interior of the evaporator. Vakil is attempting to cool the incoming liquid refrigerant stream from the condenser to prevent the formation of vapor therein prior to its evaporation, an occurrence that would reduce thermodynamic efficiency. Because Vakil does not disclose the particular form of the evaporator utilized, it cannot be ascertained with any degree of certainty whether the design of the Vakil patent lends itself to compactness.
It will therefore be appreciated that in spite of the attempts to integrate suction line heat exchangers with evaporators, significant compactness has yet to be achieved.